This invention relates to the field of micro-mechanical devices, more specifically to the methods used to remove sacrificial layers from a micro-mechanical device, and to methods used to remove sacrificial layers with a solution comprising super-critical carbon dioxide.
Micro-mechanical devices or micro-electromechanical systems (MEMS) are micron-scale devices, often with moving parts, and are fabricated using traditional semiconductor processes such as optical lithography, doping, metal sputtering, oxide deposition, and plasma etching, which have been developed for the fabrication of integrated circuits.
Micromirrors, such as the DMD(trademark) micromirror array from Texas Instruments, are a type of micro-mechanical device. Other types of micro-mechanical devices include accelerometers, pressure and flow sensors, gears and motors. Many of these devices have found wide commercial success. A novel way to fabricate micro-mechanical devices is to deposit metal layers on top of a sacrificial layer that is comprised of a solvent/resin solution such as photoresist. After the metal layers have been patterned and etched, then the sacrificial layers are removed, thus uncovering a free-standing micro-mechanical device. This process is particularly useful for the fabrication of micromirror arrays, but may also be used in the fabrication of other micro-mechanical structures.
One problem encountered during the manufacturing of micro-mechanical devices occurs when sacrificial layers are removed from the substrate to reveal a free-standing micro-mechanical device (i.e. a superstructure). Because micro-mechanical devices are very fragile, they can be easily damaged by particles, excessive heat, or even the capillary action of cleansing solvents. To avoid the problems associated with the capillary action of solvents, a plasma ash operation has been used to remove the sacrificial layers that surround devices. The plasma ash step, however, has several important drawbacks. Specifically, the plasma ash step can create a great deal of heat, which can deform or otherwise damage micro-mechanical devices. In addition, the heat can adversely affect any underlying electrical devices, which have a specific thermal budget. The plasma ash step is also problematic because it can result in the deposition/implantation of the plasma materials or other sputtered materials in the micro-mechanical devices, such as fluorine. Furthermore, plasma ash operations require the use of environmentally sensitive chemicals, such as carbon tetrafloride (CF4).
Another important problem encountered during the manufacturing of micro-mechanical devices is the low yield of the fabrication processes. The processes for fabricating micro-mechanical devices are very expensive. The back-end dicing and packaging process can be particularly costly processes. Under current manufacturing practices, most micro-mechanical devices cannot be tested until they have been fully diced and packaged. This is because the sacrificial layers that surround each device must remain in place to protect the device from the particulate contamination generated by the back-end processes. Thus, if it were possible to identify non-functional devices at an intermediate step and recoat the functional devices before the costly back-end steps are performed, then the cost per functional device could be greatly decreased. It is therefore advantageous to test the micro-mechanical devices after they have been fabricated, but before the expensive back-end process steps are performed. This intermediate testing is difficult to perform for micro-mechanical devices because once the sacrificial layers have been removed, they are very susceptible to contamination and damage. In addition, the processes of applying and removing a protective recoat layer can damage to the micro-mechanical devices.
Yet another problem associated with the removal of sacrificial layers is that many cleansing solvents cause the sacrificial layers to swell while they are removed. This swelling occurs as the sacrificial layers begin to absorb the cleansing solvents during the removal process. The swelling of the sacrificial layers can damage the micro-mechanical devices by bending or otherwise deforming them.
There is therefore a need in the art for a method for effectively removing sacrificial layers that surround a micro-mechanical device without causing the damage to the devices. Specifically, a method is needed that will not produce the capillary forces associated with most aqueous solvents or the damage that results from plasma ash operations. There is also a need in the art for a method for effectively removing a protective recoat layer that has been applied to a free-standing micro-mechanical device after the sacrificial layers have been removed. There is also a need in the art for a method for removing sacrificial or protective recoat layers from a micro-mechanical device while minimizing any swelling that may occur in these layers during the removal process.
The present invention provides an improved method for removing sacrificial layers during the process of fabricating a micro-mechanical device. The disclosed method is also suitable to remove a protective recoat layer that has been applied to a fabricated micro-mechanical device after fabrication has been complete. The method utilizes super-critical carbon dioxide as a cleansing agent for the sacrificial or protective recoat layers. Super-critical carbon dioxide has advantages over prior art cleansing methods because it has relatively low surface tension, thereby eliminating capillary forces from the removal process. Super-critical carbon dioxide is also advantageous because it does not cause any of the plasma-induced damage associated with ashing operations. Furthermore, the super-critical carbon dioxide process requires fewer environmentally sensitive chemicals than other processes.
According to one aspect of the invention, the method for cleaning sacrificial layers comprises the steps of depositing a first sacrificial layer on a substrate; curing the first sacrificial layer; removing portions of the first sacrificial layer to define a set of first via forms; depositing a first metal layer on the first sacrificial layer; removing portions of the first metal layer to define a set of first via supports; depositing a second sacrificial layer on the first metal layer; curing the second sacrificial layer; removing portions of the second sacrificial layer to define a set of second via forms; depositing a second metal layer on the second sacrificial layer; removing portions of the second metal layer to define a set of second via supports; removing the first and second sacrificial layers with a solution comprising super-critical carbon dioxide, and a solvent and/or surfactant.
In another aspect of the invention, the method for cleaning a protective recoat layer comprises the steps of depositing a first sacrificial layer on a substrate; curing the first sacrificial layer; removing portions of the first sacrificial layer to define a set of first via forms; depositing a first metal layer on the first sacrificial layer; removing portions of the first metal layer to define a set of first via supports; depositing a second sacrificial layer on the first metal layer; curing the second sacrificial layer; removing portions of the second sacrificial layer to define a set of second via forms; depositing a second metal layer on the second sacrificial layer; removing portions of the second metal layer to define a set of second via supports; recoating the micro-mechanical device with a recoat layer of sufficient thickness to completely encapsulate the micro-mechanical device; curing the recoat layer; removing the recoat layer with a solution comprising super-critical carbon dioxide and a cosolvent and/or surfactant, as described above.